If you do not want to reset all your icons, and just want to get your home launcher back, you can also try the following. alt+f2 and then type nautilus (this opens the default file manager installed with Ubuntu - I assume you did not change that). Then, you will see that the file manager icon is in your launcher. Simply right click it and select "Keep in Launcher". It will not necessarily be at the top of the Launcher. So, after you have "pinned" it to the Launcher, simply drag it with your mouse to the top of your Launcher and you should be good to go.